(1) Field of the Invention
The invention is related to a partly cruciform flexbeam of a bearingless main rotor system of a helicopter with the features of the preamble of claim 1 and to a method of manufacturing such a partly cruciform flexbeam with the features of the preamble of claim 5.
(2) Description of Related Art
During operation, the rotor blades of a helicopter are deflected in various directions and are thereby subjected to high loads in these various directions. The rotor blades must be designed to withstand these high loads while still providing the required flexibility or articulation to allow the blades to carry out flapping, lead-lag oscillating and pitch angle variation movements.
Typically, a rotor blade of a bearingless rotor includes a structural element known as a flexbeam at the inner end of the blade for connection to the rotor head. The flexbeam supports and transmits the centrifugal forces of the blade into the rotor hub. Additionally, the flexbeam includes at least portions or regions that are flexural and torsion soft or flexible to allow the blade to undergo the above mentioned movements in a flapping direction, a lead-lag direction and in a pitch angle direction. The torsion soft portion of the flexbeam is arranged within a torsion stiff control cuff or torque tube, transmitting the pitch angle control movements to the lift-generating airfoil blade portion of the rotor blade. The airfoil blade typically extends from the outboard end of the control cuff to the outermost end of the rotor blade, i.e. the blade tip. Nowadays flexbeams are made of composite materials.
The vibrations of the rotor blades, and particularly the oscillations in the lead-lag direction, must be damped by appropriate damping elements. Any softness or lack of force transmission through the blade/cuff attachment to the damping element will reduce the overall resulting damping effect.
In order to allow the flexbeam/cuff unit and/or the airfoil blade to be separately manufactured and/or replaced in the event of damage, or in order to allow the airfoil blade to be pivoted and folded relative to the flexbeam/cuff unit, it is desired to provide a separable or releasable junction between the flexbeam/cuff unit and the lift-generating airfoil blade.
The flexbeam is subject to high technical and mechanical demands, because it must reliably carry and transmit the substantially high centrifugal forces resulting during rotation of the rotor blade and it must reliably carry and transmit all bending moments from flapping and lead lag movements of the rotor blade. The rotor blade has a longitudinal main load axis next to 25% of the average airfoil chord of the blade profile, said main load axis being in practice essentially identical with the pitch axis of said rotor blade. At the level of the flexbeam said longitudinal main load axis next to 25% of the average airfoil chord of the blade profile corresponds to a longitudinal middle axis of the flexbeam.
An efficient transmission of the lead lag moments from the airfoil blade into the flexbeam/control cuff and the lift-generating airfoil blade allows high damping effectiveness of the lead lag damping elements at the rotor hub and thus allows smaller, lighter and cheaper damping elements. An efficient transmission of the lead lag moments needs a flexbeam with a distinct lead lag joint. Such a distinct lead lag joint leads to a cross section of the flexbeam at the releasable junction between the flexbeam and the lift-generating airfoil blade that is high and slim whereas the lead lag joint of the flexbeam needs to be large and low towards the rotor hub. A continuous geometry of the flexbeam from the releasable junction with the lift-generating airfoil blade to the releasable junction with the rotor hub is varying along the longitudinal middle axis of the flexbeam and said continuous geometry of the flexbeam is therefore complicated and a challenge with regard to production of the flexbeam.
The high cross section of the flexbeam at the releasable junction between the flexbeam and the lift-generating airfoil blade causes relatively high aerodynamic resistance and thus a high need of performance of the main rotor system. There are essentially two ways to reduce said high aerodynamic resistance:
1. Positioning the releasable junction between the flexbeam and the lift-generating airfoil blade as close as possible to the rotor hub, implying a short flexbeam with the consequence that the geometry of the flexbeam is further complicated in order to provide a short flexbeam with allowable shear stresses in the resins carrying most of the shear stresses of the composites.
2. Designing the releasable junction aerodynamically as favorable as possible. An aerodynamic favorable junction may be provided with a flexbeam head of bearing laminate.
The document U.S. Pat. No. 5,738,494 A discloses an optimized composite flexbeam having a plurality of adjoining regions including a hub attachment region, a blade attachment region, a pitch region, an outboard transition region disposed between and adjoining the pitch region and blade attachment regions, and an inboard transition region disposed between and adjoining the pitch and hub attachment regions. The inboard transition region includes a first transition subregion and a second transition subregion wherein the second transition subregion defines a width conic and a critical width transition subregion. Furthermore, the first and second inboard transition regions are composed of a combination of unidirectional and off-axis composite materials wherein the off-axis composite material defines a percentage of off-axis composite material and wherein the percentage in the critical transition subregion is defined by an optimized curve.
The document EP 0315962 A2 discloses a helicopter rotor blade supported by a flexbeam to be rotatable about an axis of rotation, in which a pitching motion thereof is allowable. The rotor blade is provided with a device for changing the pitch and damping the lead-lag motion thereof. The device comprises a bushing in a hole formed at the inboard end of the flexbeam, an elastomeric pivot loosely fitted in the bushing, elastomeric dampers of cylindrical shape mounted on the upper and lower surfaces of the flexbeam and coupled with the upper and lower ends of the elastomeric pivot by means of nuts, and torque arms extending through the bushing and the elastomeric dampers and having the central portion thereof connected to central shafts of the elastomeric pivots. Each of the pitch sleeves has both ends outwardly projecting beyond the elastomeric dampers and secured to a pitch sleeve which encloses the flexbeam. Therefore, the relative position between the pitch sleeve and the elastomeric pivot does not change even when lead-lag motion is imparted to the rotor blade.
U.S. Pat. No. 5,096,380 A discloses a flexbeam for a bearingless helicopter rotor with a composite beam, composed of unidirectional fibers bound in an epoxy matrix, having ribs, composed of unidirectional fibers bound in a urethane matrix, bonded to each horizontal face at the section of the beam which accommodates lead-lag torsion.
The document U.S. Pat. No. 4,427,340 A discloses helicopter rotors and more particularly rotor mounting involving a composite fiber-reinforced unitary yoke with resilient in plane restraints.
The document EP 0019041 A1 discloses a bearingless rotor, especially for helicopters, whereby the rotor has at least two rotor blades, each of which is secured to the rotor hub through a connecting element that functions as a flapping hinge, as a lead-lag hinge and as a torsion hinge. The connecting element is made of fiber compound material as an integral part of the rotor blade and extends between a blade root of the rotor blade and the rotor hub. The cross section of the connecting element can be cruciform with components that enclose right angles and that comprise shearing webs located in internal zones and tension belts that are located in outer zones. Furthermore, slots extending in the shearing webs of the components from the outer edge radially toward a shearing center or middle axis of the connecting element can be provided. Even in the tension belts, such slots may be provided extending toward or substantially toward the shearing center.
The flexbeams of the state of the art are relatively long with simple cross sections to allow simple tools for production at the cost of efficient lead lag kinematics.